State of the planet· The columns of time

Sturtian Snowball Earth

717 Ma → 660 Ma

From 717 to 660 million years ago, the Sturtian glaciation covered the planet for more than fifty million years, down to the equator.

Planetary glaciation of more than fifty million years, reaching equatorial latitudes.

What the evidence shows

Ice reaches the tropics, and both its establishment and its retreat are dated with a precision rare at this chronological depth: onset at 716.5 million years to within 200,000, globally synchronous deglaciation at 660.98 million to within 740,000, fifty-six million years later. The synchrony is the decisive result: a simultaneous retreat across the globe characterises a planetary event and not a succession of regional ones.

Bounds and conventions

The dating is exceptionally tight for this chronological depth, and that precision proceeds not from the glacial debris, which is hard to date, but from black shales above and below, whose rhenium-osmium pair supplies a clock. Those shales report neither the thickness of the ice nor whether open water persisted: the clock places the ice in time without describing it.